Glycerol, derived from the methanolysis of plant oils at the same time as methyl esters, constitutes a renewable starting material which meets the criteria associated with the new concept of “green chemistry”.
It has been known for a long time that glycerol can lead to the obtaining of acrolein according to the following dehydration reaction:CH2OH—CHOH—CH2OHCH2═CH—CHO+2H2O
As a general rule, in order to obtain acrolein, it is necessary to use a sufficient temperature, and/or partial vacuum to shift the reaction. The reaction can be carried out in a liquid phase or in a gas phase and it is generally catalyzed by acids.
Various methods for producing acrolein from glycerol have been the subject of patent applications; reference may in particular be made to documents FR 695931; U.S. Pat. No. 5,387,720; WO 06/087083; WO 06/087084; WO 06/136336.
The glycerol is generally used in the form of aqueous solutions, which are more economical. However, the glycerol solution should not be too dilute owing to the energy cost generated by the evaporation of the aqueous solution of glycerol.
In methods for dehydrating glycerol so as to give acrolein in the gas phase, it is necessary to evaporate the glycerol at a high temperature, generally above 220° C. It is, however, well known that glycerol is not stable at high temperature, in particular at above 220° C.
In the methods proposed for evaporating aqueous solutions of glycerol, it is therefore very critical to control the temperature since certain undesirable reactions can take place, such as the formation of nitrogenous compounds by degradation of protein material present in the crude glycerol, or the formation of glycerol ethers or of polyglycerol. It is therefore important to limit the residence time of the glycerol at high temperature, and also this temperature.
The solution conventionally developed is to use a low glycerol partial pressure, for example with glycerol solutions that are sufficiently diluted in water, or at reduced pressure, in order to decrease the temperature necessary for evaporation of the glycerol. However, this type of solution results in low yields since the partial reactive pressure is necessarily low.
The methods for evaporation conventionally used do not therefore make it possible to have high glycerol partial pressures in the vapor phase. Moreover, it is often necessary to combine the glycerol evaporation step with a pretreatment for eliminating impurities, such as sodium chloride, sodium sulfate, non-glycerin organic matter, or methanol, that may be present in the aqueous solution of glycerol.
In patent application FR 2 913 974, the applicant company describes a single-step method for vaporizing an aqueous glycerol solution and simultaneously eliminating the impurities present in the solution or generated during the evaporation. This method consists in vaporizing the aqueous glycerol solution in contact with a fluidized bed containing an inert solid maintained at a temperature sufficient to allow instantaneous vaporization of the glycerol and of the water. The impurities present in the aqueous solution are simultaneously eliminated since the fluidized bed technique makes it possible to continuously withdraw part of the solid in order to regenerate it ex-situ. The glycerol vapors obtained according to this method can then be used directly in a method for producing acrolein in the gas phase.
However, this prior glycerol vaporization step can prove to be expensive if the glycerol is used in the form of a dilute aqueous solution.